There has been considerable recent interest in the fabrication of rare earth doped thin films for optically active waveguides for integrated optics applications. Since rare earth ions exhibit a characteristic intra-4f shell luminescence emission that is both nearly host and temperature independent, rare earth doped ferroelectric oxides have been of partcular interest as offering the possibility of simple optical devices that take advantage of the electro-optical and nonlinear optical (NLO) properties of ferroelectric oxides as well as the optical gain of the rare earth ions.
Optical devices, such as self-frequency-doubled, self-Q-switched, and self-modulated lasers in addition to amplified integrated optical circuits with no insertion losses are possible using rare earth doped ferroelectric oxides. Erbium-doped ferroelectric oxides are of special interest as optically active components due the characteristic Er.sup.3+ emission at 1.54 microns, which corresponds to the minimum loss in silica based optical fibers. For example, planar waveguides and devices, including self-frequency doubled devices and lasers operating at near 1.54 microns have been fabricated from rare earth doped lithium niobate (LiNbO.sub.3) bulk single crystals. Lithium niobate, however, exhibits several inherent limitations. First, the solubility of erbium ions (Er+) in the lithium niobate host material appears to be relatively low. Second, photo-refractive optical damage of the lithium niobate host can limit the efficiency and usefulness of lithium niobate based waveguides and optical devices. While doping the lithium niobate host with MgO is known to reduce the photorefractive damage problem for other applications, the presence of a rare earth dopant in the lithium niobate host may reduce the beneficial effect achieveable by the MgO dopant. Third, optical waveguides comprising erbium doped lithium niobate can only be made from bulk single crystal material, which is itself difficult to make, and requires a slow, costly diffusion or ion implanation treatment to render it waveguiding and also to include the erbium dopant therein.
An object of the present invention is to provide a thin film optical working medium that provides one or more of the non-linear, electro-optic and other properties associated with certain ferroelectric oxides and that includes a rare earth dopant incorporated in-situ in the host ferroelectric oxide.
Another object of the present invention is to provide an optical device that comprises an optically active or passive rare earth doped barium titanate thin film optical component that overcomes the disadvantages of the aforementioned bull erbium doped lithium niobate optical component.
Still another object of the present invention is to provide a method of in-situ doping of a ferroelectric oxide thin film, such as barium titanate, with rare earth as the film is deposited by metalorganic chemical vapor deposition on a substrate.